Phase noise is an inherent problem in the design of radio circuitry. It is due mainly due to noise generated by active devices used with a tuning circuit for sustaining oscillations in the oscillator circuitry. It is considered to be due to modulation due to the 1/f baseband noise spectrum of the nonlinear transfer characteristic and limiting behavior of the active device. This phase noise is conventionally reduced by the filtering effect of a resonant tank circuit. Its effectiveness in reducing phase noise is dependent upon its loaded Q which is indicative of energy lost per cycle relative to total stored energy in the tank circuit. The energy lost per cycle is that energy dissipated by its reactive elements. The energy output is used to promote the oscillation of the oscillator. Losses due to tuning elements of the tank circuit such as varactor diodes, such as are used in voltage controlled oscillators, are a primary factor in reducing its Q. Loaded Q of the tank circuit is a measure of its ability to successfully filter the phase noise.
Since the ability of resonant tanks to provide high Q decreases with increasing frequency, it becomes increasingly difficult to provide acceptable levels of phase noise at the high RF frequencies required for new wireless communication systems.
Phase noise may be reduced by increasing the Q of the resonant tank circuit. Many methods have been tried to increase the Q of the resonant circuit including insertion of amplifier circuitry in the resonant circuit, isolating the fundamental frequency from the output and compensating for oscillator frequency changes.
The effectiveness of filtering however decreases with frequency of operation as the Q of the tuned circuit decreases resulting to high energy dissipation for cycle. Various implementations of these methods are disclosed in the Art.
An article "Injection Locking of Microwave Solid-State Oscillators" by K. Kurokawa in Proceedings of the IEEE; Vol 61, No 10 (October, 1973) discusses analysis of injection locking of microwave oscillators. A discussion of a random noise vector of oscillation and relation of noise to Q is also included.
U.S. Pat. No. 5,625,324 discloses an Ultra Low Noise Frequency Generator producing a sum of Plural Signal Sources. The signal frequencies of the plural sources are summed to produce a new frequency signal having an output with low noise.
U.S. Pat. No. 5,379,309 illustrates injection locking of two independent laser oscillators to a mode locked oscillator. The objective is a low phase noise oscillator output.
U.S. Pat. No. 5,059,927 discloses a 60 GHz microwave oscillator with reduced phase noise. Phase noise reduction is achieved by selected terminations of the oscillator circuit.
U.S. Pat. No. 5,019,791 concerns flicker (1/f) noise suppression by providing voltage compensation to the tuned circuit varactor to compensate for changes in oscillator frequency.